The synapsins are a family of neuron-specific, vesicle-associated phosphoprotein which are important for the regulation of neurotransmitter release from mature neurons, and may also play a critical role in synaptogenesis. The phosphorylation-dependent cross-linking by synapsin I of synaptic vesicles and the actin based cytoskeleton is believed to regulate the availability of vesicle for exocytosis. Proline-directed protein kinases represent a broad family of kinase, including cyclin-dependent protein kinases (cdks) and mitogen-activated protein kinases (MAP kinases). These kinases are known to play an essential role in cell division, growth and differentiation. Recently, we have demonstrated that synapsin I is a downstream effector of the MAP kinase pathway, and is also phosphorylated by cdc2 and cdk 5, a cdk that is enriched in adult, post-mitotic neurons. Synapsin II, the second member of the synapsin family, can also be phosphorylated in vitro by MAP kinase and cdc2 kinase. We wish to identify the cdk5-dependent site in synapsin I and each proline-directed site in synapsin II. Since the amounts of synapsin II that are available are very limited, even when the protein is produced by recombinant technology, we hope to take advantage of mass spectrometric analysis to identify these sites.